The present invention relates to a silica-based film, a method of forming the same, a composition for forming an insulating film for a semiconductor device, an interconnect structure, and a semiconductor device.
A silica (SiO2) film formed by a vacuum process such as a CVD process has been widely used as an interlayer dielectric for a semiconductor device used in a large-scale integrated circuit (LSI) or the like. In recent years, in order to form an interlayer dielectric having a more uniform thickness, a spin-on-glass (SOG) film, which is a coating-type insulating film containing an alkoxysilane hydrolysate as the major component, has also been used. Along with an increase in the degree of integration of the LSI, a low-dielectric constant interlayer dielectric containing organic silica sol represented by methyl silsesquioxane (MSQ) has also been developed (U.S. Pat. Nos. 6,235,101, 6,413,647, and 6,495,264).
The organic silica sol is cured by causing the silanol group in the sol to undergo a dehydration-condensation reaction by heating at 350 to 500° C., whereby an insulating film exhibiting a dielectric constant, mechanical strength, and chemical durability suitable as an interlayer dielectric for a semiconductor device can be formed. However, since the reaction of the organic silica sol is a solid-phase reaction, dehydration-condensation does not rapidly proceed due to diffusion control. Therefore, it is necessary to heat the organic silica sol for a long time (e.g. 30 minutes at least; usually one hour or more). In order to perform such a long heat treatment, a batch-type heat treatment furnace capable of treating 50 to 150 wafers at a time has been used to treat a spin-on low-dielectric-constant interlayer dielectric. A semiconductor device which mainly requires a low-dielectric-constant interlayer dielectric is a semiconductor device in the logic device field. However, a logic device interconnect manufacturing step has been tending toward a single-wafer process in which a wafer is rapidly processed one by one. This is because a mainstream logic device such as an ASIC or a custom IC is manufactured in a high-variety low-volume production process. Specifically, the single-wafer process has become the mainstream manufacturing process in order to improve the degrees of freedom of the manufacturing steps.
As a method for rapidly curing a low-dielectric-constant interlayer dielectric composition containing organic silica sol as the major component while improving the strength, a method using electron beams has been proposed (U.S. Pat. No. 6,204,201 and European Patent No. 1122770). This method causes not only a silanol condensation reaction, but also causes decomposition and activation of an organic group in the organic silica-based film to introduce a crosslinked structure such as Si—CHx—Si. A film exhibiting low hygroscopicity and excellent mechanical strength can be obtained by applying electron beams usually within five minutes, whereby the single-wafer processing can be performed. On the other hand, accumulation of an electric charge due to electron beam application may damage the transistor structure in the LSI. Therefore, arguments exist for and against curing a low-dielectric-constant interlayer dielectric composition using electron beams (E. Mickler et al. Proceedings of the International Interconnect Technology Conference, p190, 2004., (Miyajima, et al. Proceedings of the International Interconnect Technology Conference, p. 222, 2004).
A method using ultraviolet radiation is considered as a method for rapidly curing a low-dielectric-constant interlayer dielectric composition containing organic silica sol as the major component without using electron beams. Now, technologies other than the LSI interlayer dielectric technology are considered below. A technology of gelling silica sol by adding a photoacid generator or a photobase generator, which generates an acid or a base upon exposure to ultraviolet radiation, to silica sol and an alkoxysilane to promote a condensation reaction of a silanol and an alkoxide has been known as an optical sol-gel technology, and has been applied to formation of an optical waveguide or the like (e.g. Japanese Patent Application Laid-Open No. 2000-109695). A silica film cured by using a photoacid generator or a photobase generator generally exhibits high hygroscopicity due to a large amount of residual silanol. As a result, the resulting film has a high dielectric constant. The hygroscopicity due to the residual silanol may be reduced by gelling the silica sol by applying ultraviolet radiation and heating the resulting product at about 250 to 500° C. for a predetermined time or more (usually 30 minutes or more). However, this process does not achieve an improvement over the above-described silica film thermal curing method. Moreover, a composition containing a photoacid generator or a photobase generator cannot satisfy the quality as an insulating film for an LSI semiconductor device for which high insulation reliability is demanded, since the photoacid generator, the photobase generator, or an acidic or basic substance generated by the photoacid generator or the photobase generator functions as a charge carrier to impair the insulating properties or causes an interconnect metal to deteriorate.
A siloxane compound is highly transparent to ultraviolet radiation, and has been vigorously studied as a backbone of an F2 photoresist using ultraviolet radiation having a wavelength of 157 nm (e.g. Japanese Patent Application Laid-Open No. 2002-288268). This technology uses a siloxane backbone, but is basically based on the principle of a chemically-amplified photoresist using a KrF or ArF light source. Specifically, a photoacid generator generates an acidic substance upon exposure to ultraviolet radiation, and a chemical bond cleaved by an acid produces a functional group, such as a carboxylic acid, which is readily dissolved in a basic developer. Therefore, this technology does not promote a crosslinking reaction of silica sol by ultraviolet radiation.
The surface of the organic silica-based film cured by applying heat, electron beams, or the like has high hydrophobicity. In order to decrease the surface hydrophobicity, ultraviolet radiation may be applied to the organic silica-based film (e.g. U.S. Pat. No. 6,383,913, Japanese Patent Application Laid-Open No. 63-248710, Japanese Patent Application Laid-Open No. 63-289939, Kokoku publication No. 8-29932, Japanese Patent Application Laid-Open No. 2001-110802). These technologies are characterized in that the surface of the organic silica-based film is oxidized by ozone produced by applying ultraviolet radiation in air so that the hydrophobic surface is changed into a hydrophilic surface having high reactivity, such as a silanol. This modification treatment is mainly performed in order to improve adhesion to a film deposited as the upper layer.
As described above, a technology of applying a polysiloxane resin solution or an organic silica sol solution to a substrate and applying ultraviolet radiation to the resulting film has been widely studied. However, a technology which uses ultraviolet radiation for curing organic silica sol in order to form an interlayer dielectric for an LSI semiconductor device is limited. Japanese Patent Application Laid-Open No. 3-30427, Japanese Patent Application Laid-Open No. 1-194980, International Patent Application No. WO 03/025994, and U.S. patent application Ser. No. 2004/0058090 disclose such limited related-art technologies.
Japanese Patent Application Laid-Open No. 3-30427 discloses a technology in which a solution prepared by dissolving a tetraalkoxysilane (e.g. tetraethoxysilane: TEOS) in collodion is applied to a semiconductor substrate, and ultraviolet radiation is applied to the solution in a nitrogen atmosphere to obtain a silicon dioxide film at a low temperature. The feature of this technology is that highly volatile TEOS is fixed using the collodion, and decomposition of the collodion and dehydration and condensation of TEOS are promoted by applying ultraviolet radiation. Japanese Patent Application Laid-Open No. 1-194980 discloses a technology in which an organosiloxane resin is applied to a substrate, ultraviolet radiation having a dominant wavelength of 254 nm is applied to the resin at a temperature of 200° C. or less to oxidize the surface of the organosiloxane film by ozone produced by ultraviolet radiation, and the oxidized film is heated at 400° C. or more, particularly about 900° C. to obtain a dense silicon dioxide film.
International Patent Application No. WO 03/025994 and U.S. patent application No. 2004/58090 disclose a technology of curing hydrogenated silsesquioxane (HSQ) or MSQ by applying ultraviolet radiation. In this technology, ultraviolet radiation is applied to HSQ or HSQ/MSQ cocondensate in the presence of oxygen so that active oxygen (e.g. ozone) produced in the system promotes oxidation of Si—H in HSQ to form a silica film containing a large amount of SiO2 bond. These references describe that this technology is also effective for curing MSQ in the presence of oxygen rather than the absence of oxygen. Therefore, it is estimated that the SiO2 bond formed by active oxygen is the principal mechanism of the crosslinking reaction. The feature of this technology is the use of ultraviolet radiation, since it is impossible to form the SiO2 bond in a short time using other curing methods. However, while a silica film formed according to this technology has a high modulus of elasticity and high hardness due to an increase in the amount of the SiO2 bond, the moisture absorption and the dielectric constant are increased due to an increase in hydrophilicity of the film. A film having high hygroscopicity generally tends to be damaged by reactive ion etching (RIE) performed in the processing of an interlayer dielectric of a semiconductor device, and exhibits insufficient chemical resistance against a wet cleaning liquid. This tendency significantly occurs in a low-dielectric-constant interlayer dielectric having a porous structure with a dielectric constant k of 2.5 or less. Therefore, (a) an organic silica sol composition which does not include an ionic substance such as a photoacid generator, photobase generator, or photosensitizer, a charge carrier, or a corrosive compound generation source, and can be cured in a short time, (b) a method for curing an organic silica-based film which does not cause damage to a transistor structure and enables single-wafer processing, (c) an organic silica-based film which does not exhibit hygroscopicity and exhibits high hydrophobicity, and (d) an organic silica-based film which exhibits such mechanical strength that the organic silica-based film can withstand formation of a copper damascene structure, are demanded as a low-dielectric-constant interlayer dielectric for an LSI semiconductor device along with a method of forming the same.
An organic silica sol composition for a low-dielectric-constant insulating film used for a semiconductor device is generally designed so that the composition of the organic silica sol is controlled so that an organic silica film obtained by curing the composition by heating has a high modulus of elasticity, taking into consideration the yield in a step in which a dynamic stress occurs, such as chemical mechanical polishing (CMP) or packaging (e.g. U.S. Pat. No. 6,495,264). In more detail, the organic silica sol composition is designed so that the absolute crosslink density in the silica film is increased by increasing the amount of silicon atom which bonds to four oxygen atoms (hereinafter called “component Q”) in the organic silica sol to usually 40 mol % or more. The crosslink density is increased by increasing the amount of component Q, whereby a film having a high modulus of elasticity and high hardness can be obtained. However, it is difficult to cause the crosslink site (silanol) of the component Q to completely react. If the amount of component Q is increased to a large extent, the amount of residual silanol is increased after thermal curing, whereby the resulting film exhibits hydrophilicity and high hygroscopicity. In order to compensate for this drawback, cocondensation with an alkoxysilane having a hydrophobic group such as a methyltrialkoxysilane is carried out using a basic catalyst (e.g. ammonia or tetraalkylhydroxyammonium) to produce a sol having a high degree of condensation to reduce the absolute amount of silanol in the sol (U.S. Pat. No. 6,413,647), or the sol having a high degree of condensation is subjected to an additional hydrophobic treatment (Japanese Patent Application Laid-Open No. 2004-59737 and Japanese Patent Application Laid-Open No. 2004-149714). However, since the organic silica sol containing a large amount of component Q has a low molecular chain mobility due to high crosslink density, the diffusion barrier during the solid-phase reaction is very high. The condensation reaction is not promoted even if the organic silica sol containing a large amount of component Q is cured at 400° C. while applying ultraviolet radiation within five minutes. Therefore, a curing time of 30 minutes or more is required for causing the organic silica sol to react.